GRAVITATIONAL METHODS 



417 



around the station to be corrected. The points may also be taken on the 

 corners of a square (w = 4), a hexagon (n = 6), or a decagon (n =10), 

 etc. The dimensions of these figures may be varied. Figure 247 shows a 

 conventional gravity survey map by Griffin, covering the South Houston 

 Field and the adjacent area, and contoured at an interval of 0.2 millidyne. 



SOUTH HOUSTON FIELD GRAVITY MAP 

 »HARRIS COUNTY TEXAS 

 MAP SCALE 1 MILE = ■ 



Cl-0 2 MO 



Fig. 247. — Conventional gravity map of South Houston field 

 and adjacent area. Contour interval 0.2 millidyne. (W. R. Griffin, 

 Geophysics, Vol. 14, No. 1, Jan., 1949.) 



Figure 248, covering the same locality, illustrates the procedure under 

 consideration. On this map is superimposed an octagon net which was 

 used to calculate the average gravity value or regional effect, applicable 

 at the center point of the net. Each octagon in the net permits 8 gravity 

 map points to be averaged for the radius or outdistance selected. 



It was found that the maximum variation from averaging 4 points 

 (using a square) as against 8 points (using an octagon) was about 0.05 

 millidynes. In another test case, the difference between averaging 4 points 

 (square) and 10 points (decagon) around the station was 0.10 milUdynes. 

 The number of points to be averaged usually is not a critical factor in cal- 

 culating the regional correction. 



Although the geometric pattern which controls the number of points 

 averaged may not be critical, the dimensions of the figure are quite impor- 

 tant. In the first test case, if the outer radius of the square (or 4-point 



